1. Technical Field
The subject matter described herein relates to solid-state and “nearly solid state” devices for controlling the flow of light and radiant heat through downconversion and selective reflection. The technology has particular, but not exclusive, application in passive or active temperature-regulating films, materials and devices, especially as construction materials.
2. Description of the Related Art
Photodarkening materials have been used for decades, for example in sunglass lenses, to selectively attenuate incoming light when stimulated by ultraviolet (UV) radiation. When incorporated into windows, such materials can be used to regulate the internal temperature of a structure by darkening to attenuate bright sunlight and by becoming transparent again to allow artificial light or diffuse daylight to pass through unimpeded. Such systems are passive and self-regulating, requiring no external signal other than ambient UV light in order to operate. However, because they are controlled by ultraviolet rather than by temperature, such systems are of limited utility in temperature-regulating applications.
Electrodarkening and photodarkening materials attenuate incoming light primarily through absorption rather than reflection, meaning they will heat up when exposed to bright light. This creates a conductive heat flux which offsets the reductions in radiative transmission and thus places significant limits on their ability to regulate temperature.
The process of absorbing one wavelength of light and emitting another, longer wavelength of light is known as downconversion. This process occurs in a number of naturally occurring fluorescent and phosphorescent materials, including phosphorus. Blackbody radiation from an energy-absorbing material is a form of downconversion as well. Downconversion also occurs in semiconductor materials, which absorb energy over a wide band of wavelengths and emit energy in a much narrower band, centered on the bandgap energy of the material, through a process known as photoluminescence. A downconverter can easily be fashioned from a piece of bulk semiconductor.
The fabrication of very small structures to exploit the quantum mechanical behavior of charge carriers, e.g., electrons or electron “holes” is well established. Quantum confinement of a carrier can be accomplished by a structure having one or more dimensions less than the quantum mechanical wavelength of the carrier. Confinement in a single dimension produces a “quantum well,” and confinement in two dimensions produces a “quantum wire.”
A quantum dot is a structure capable of confining carriers in all three dimensions. Quantum dots can be formed as particles, with dimensions in all three directions of less than the de Broglie wavelength of a charge carrier. Quantum confinement effects may also be observed in particles of dimensions less than the electron-hole Bohr diameter, the carrier inelastic mean free path, and the ionization diameter, i.e., the diameter at which the quantum confinement energy of the carrier is equal to its thermal-kinetic energy. It is postulated that the strongest confinement may be observed when all of these criteria are met simultaneously. Such particles may be composed of semiconductor materials (for example, Si, GaAs, AlGaAs, InGaAs, InAlAs, InAs, and other materials), or of metals, and may or may not possess an insulative coating. Such particles are referred to in this document as “quantum dot particles.”
Quantum dots can have a greatly modified electronic structure from the corresponding bulk material, and therefore different properties. Because of their unique properties, quantum dots are used in a variety of electronic, optical, and electro-optical devices. Quantum dots are currently used as near-monochromatic fluorescent light sources, laser light sources, light detectors including infrared (IR) detectors, and highly miniaturized transistors, including single-electron transistors.
The embedding of metal and semiconductor nanoparticles inside bulk materials (e.g., cadmium sulfide particles as a colorant in ornamental crystal) has been practiced for centuries. However, an understanding of the physics of these materials has only been achieved comparatively recently. These nanoparticles are quantum dots with characteristics determined by their size and composition. These nanoparticles serve as dopants for the material in which they are embedded to alter selected optical or electrical properties. The “artificial atoms” represented by these quantum dots have properties which differ in useful ways from those of natural atoms. However, it must be noted that the doping characteristics of the quantum dots are fixed at the time of manufacture and cannot be adjusted thereafter.
Leatherdale et al., “Photoconductivity in CdSe Quantum Dot Solids,” Physics Review B (15 Jul. 2000), describe, in detail, the fabrication of “two- and three-dimensional . . . artificial solids with potentially tunable optical and electrical properties.” These solids are composed of colloidal semiconductor nanocrystals deposited on a semiconductor substrate. The result is an ordered, glassy film composed of quantum dot particles, which can be optically stimulated by external light sources or electrically stimulated by electrodes attached to the substrate to alter optical and electrical properties.
U.S. Pat. No. 5,881,200 to Burt discloses an optical fiber (1) containing a central opening (2) filled with a colloidal solution (3) of quantum dots (4) in a support medium. The purpose of the quantum dots is to produce light when optically stimulated, for example, to produce optical amplification or laser radiation. The quantum dots take the place of erbium atoms, which can produce optical amplifiers when used as dopants in an optical fiber. The characteristics of the quantum dots can be influenced by the selection of size and composition at the time of manufacture. Although this device has an input or source path and an output or drain path, it does not have means of external control, and so is not a “switch” in any meaningful sense. As such, it does not prevent or regulate the flow of light energy through the fiber.
Goldhaber-Gordon et al., “Overview of Nanoelectronic Devices,” Proceedings of the IBEE, Vol. 85, No. 4, (Apr. 1997), describe what may be the smallest possible single-electron transistor. This consists of a “wire” made of conductive C6 benzene molecules with a “resonant tunneling device,” or “RTD,” inline that consists of a benzene molecule surrounded by CH2 molecules, which serve as insulators. The device is described, perhaps incorrectly, as a quantum well (rather than a quantum dot) and is intended as a switching device transistor rather than a confinement mechanism for charge carriers. However, in principle the device should be capable of containing a small number of excess electrons and thus serving as a quantum confinement device. Thus, the authors remark that the device may be “much more like a quantum dot than a solid state RTD.” (See p. 19.)
U.S. Pat. No. 6,512,242 to Fan et al. describes a device for producing quantum effects comprising a quantum wire (504), energy carried along the quantum wire under voltage control, and quantum dots (502, 503) near the quantum wire that hold energy. The quantum wire transports electrons into and out of a quantum dot or plurality of quantum dots through “resonant tunneling.” As described by Fan et al., the quantum dots serve as “resonant coupling elements” that transport electrons along the quantum wire acting as an electronic waveguide or between different ports on the same waveguide. In other words, the quantum dots serve as a kind of conduit.
U.S. patent application publication no. 2002/0079485A1 by Stinz et al. discloses a “quantum dash” device that can be thought of as a non-spherical, non-radially-symmetric quantum dot particle with elongated axes, or as a short, disconnected segment of quantum wire. In this sense, quantum dashes are merely a special class of quantum dot particles. As described by Stinz et al., pluralities of the quantum dash devices are embedded at particular locations inside a solid material to enhance the excitation of laser energy within the material. The resulting structure is a “tunable laser” with an output frequency that can be adjusted over a narrow range. This tuning is accomplished through “wavelength selective feedback” using an external optical grating to limit the input light frequencies that can reach the dashes inside the material. The publication notes that “an ensemble of uniformly sized quantum dashes that functioned as ideal quantum dots would have an atomic-like density of states and optical gain.” Stinz et al. relies on the exact geometry and composition of the semiconductor material to produce quantum dashes of a particular size and shape. Therefore, selection of the available quantum states is achieved exclusively at the time of manufacture, “with a variety of length-to-width-to-height ratios, for example, by adjusting the InAs monolayer coverage, growth rate, and temperature.” The energy affects all the quantum dashes equally, along with the surrounding material in which they are embedded, and if the surrounding material is opaque, then photon energy cannot reach the quantum dashes at all. Again, this device is not an optical switch.
U.S. Patent Application Publication No. 2002/0114367A1 by Stinz et al. discloses “an idealized quantum dot layer that includes a multiplicity of quantum dots embedded in a quantum well layer sandwiched between barrier layers.” Similarly, U.S. Pat. No. 6,294,794 B1 to Yoshimura et al. discloses “a plurality of quantum dots in an active layer such that the quantum dots have a composition or doping modified asymmetric in a direction perpendicular to the active layer.” These quantum dot particles are simply embedded in an optical crystal. A similar quantum dot layer structure is disclosed in U.S. Pat. No. 6,281,519 B1 to Sugiyama et al.
McCarthy, et al., in U.S. Pat. No. 6,978,070, discloses in detail a plurality of bank-addressable quantum dot devices which can be used as programmable dopants to alter the bulk electrical, optical, thermal, magnetic, chemical, and mechanical properties of a substrate (whether cylindrical, flat, or some other shape) in a controlled and repeatable way. This control could take place not only at the time of fabrication of the material, but also in real time, i.e., at the time of use, in response to changing needs and conditions.
McCarthy et al, in U.S. Patent Application Publication No. 2006/0011904, discloses a layered composite film incorporating quantum dots as programmable dopants. A means is described in detail for controlling large numbers of quantum dots in order to affect the bulk properties of a substrate near its surface. The device may incorporate switches in order to turn power on and off to control wires or control wire branches, but these switches are not thermally controlled. The authors also note that the device “can . . . be used as a solid-state thermal switch, i.e., it can be switched between thermally conductive and thermally insulating states, forming the thermal equivalent of an electronic transistor or rheostat.” However, the configuration of such a thermal switch is not specified, e.g., the input and output paths are not drawn or described, although the source, drain, and gate of the switches (122) in the control wires are clearly shown.
Harrison, “Quantum Wells, Wires, and Dots,” John Wiley & Sons, Ltd. (2000) notes the existence of a “two dimensional electron gas field effect transistor (TEGFET) . . . a type of High Electron Mobility Transistor (HEMT) designed to exploit the high in-plane (x-y) mobility which “arises when a . . . heterojunction is modulation-doped.” This design includes the one-dimensional quantum confinement of carriers (i.e., confinement along the z-axis) which can occur at a heterojunction (i.e., at the interface between two electrically dissimilar materials). However, since carriers are only free to travel in the x-y plane, and since there is no quantum confinement in the x or y direction, the one-dimensional quantum confinement is incidental rather than exploited, and does not play a necessary role in the functioning of the device. While this device is certainly a switch, it is neither optical in nature nor thermally controlled.
Harrison also discloses an effect known as the Quantum Confined Stark Effect, wherein an electric field is applied perpendicular to a quantum well to affect the energy level of the carriers confined within it. While this is known to have a slight effect on the absorption spectrum of the quantum well, the effect is exploited in sensors rather than in switches. In addition, Harrison does not state or imply that the Stark Effect has ever been used to modify the behavior of a TEGFET device or any other type of switch.
There is one other type of switch that relies on quantum confinement: the single-electron transistor or SET. This consists of a source (input) path leading to a quantum dot particle or quantum dot device, and a drain (output) path exiting, with a gate electrode controlling the dot. With the passage of one electron through the gate path into the device, the switch converts from a conducting or closed state to a nonconducting or open state, or vice-versa. However, SETs are not designed to control the flow of thermal or optical energy and do not incorporate optical downconverters or bandblock filters.
Thermal switches also exist, which allow the passage of heat energy in their ON or closed state, but prevent it in their OFF or open state. However, these switches are mechanical relays, which rely on contact between two conducting surfaces (typically made of metal) to enable the passage of heat. When the two surfaces are withdrawn, heat energy is unable to conduct between them except through the air gap. If the device is placed in vacuum, heat conduction is prevented entirely in the open state. Another type of thermal switch involves pumping a gas or liquid in or out of a chamber. When the chamber is full, it conducts heat. When empty, there is no conduction. Notably, these devices are not solid-state, not multifunctional, not programmable, and do not rely on quantum confinement for their operation.
Optical switches also exist. Light can be blocked by optical filters which absorb or reflect certain frequencies of light while allowing others to pass through. Shortpass and longpass filters may be used or a narrow range of frequencies can be blocked by a notch filter or bandblock filter. Some filters today also incorporate quantum wells, quantum wires, or quantum dot particles.
The addition of a mechanical shutter can turn an otherwise transparent material—including a filter—into an optical switch. When the shutter is open, light passes through easily. When the shutter is closed, no light passes. If the mechanical shutter is replaced with an electrodarkening material such as a liquid crystal, then the switch is “nearly solid state,” with no moving parts except photons, electrons, and the liquid crystal molecules themselves. Other electrodarkening materials, described for example in U.S. Pat. No. 7,099,062 to Azens et al., can serve a similar function. It will be clear to a person of ordinary skill in the art that these optical filter/switch combinations are not passive, but must be operated by external signals.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded as subject matter by which the scope of the invention is to be bound.